Stator construction for an axial flow compressor

ABSTRACT

A multistage compressor stator for an axial flow gas turbine engine is disclosed. A plurality of vane assemblies is mounted in a circumferential track of the case inner wall. Each vane assembly includes one or more stator vanes extending radially inward from the case inner wall into the axial flow path of the working medium, and an arcuate retainer which slidably engages the base portion of each vane. During operation of the engine the vanes are subject to vibration which is dissipatively transferred to both the working medium and the compressor case.

[ 1 Nov. 11, 1975 STATOR CONSTRUCTION FOR AN AXIAL FLOW COMPRESSORInventors: Richmond G. Shuttleworth, Vernon;

Donald L. Williams, Manchester, both of Conn.

United Technologies Corporation, Hartford, Conn.

Filed: July 29, 1974 Appl. No.: 492,574

Assignee:

US. Cl. 415/217; 415/219 R Int. Cl. F01D 25/24; F04D 19/02 Field ofSearch 415/216, 217, 218, 219 R,

References Cited UNITED STATES PATENTS 8/1961 Bean et a1 415/218 2/1967Bobo 415/216 6/1967 Bobo 415/218 11/1973 Canova et al.v 415/216 FOREIGNPATENTS OR APPLICATIONS 918.522 2/1963 United Kingdom 415/217 1.021.4953/1966 United Kingdom 415/218 1.252.179 12/1960 France 415/217 PrimaryE.\'aminer-Henry F. Raduazo Attorney, Agent, or FirmRobert C. Walker [57 ABSTRACT A multistage compressor stator for an axial flow gas turbineengine is disclosed. A plurality of vane assemblies is mounted in acircumferential track of the case inner wall. Each vane assemblyincludes one or more stator vanes extending radially inward from thecase inner wall into the axial flow path of the working medium, and anarcuate retainer which slidably engages the base portion of each vane.During operation of the engine the vanes are subject to vibration whichis dissipatively transferred to both the working medium and thecompressor case.

9 Claims, 6 Drawing Figures US. Patent Nov. 11, 1975 Sheet 1 of33,918,832

Shet 2 of 3 US. Patent Nov. 11, 1975 US. Patent Nov. 11, 1975 Sheet 3013 3,918,832

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STA'IOR CONSTRUCTION FoRAN' AXIAL FLOW COMPRESSOR BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relatesv to gasturbine engines and more specificallyto compressor stators for gasturbine engines. s M

2. Description of the Prior Art Gas turbine engines are designed andconstructed to provide adequate structural integrity of the individualcomponents while maintaining acceptable levels of aer odynamicperformance. In the compressor section a stator encases the rotorassembly. The tips of the compressor blades, which extend radiallyoutward from the rotor into the flow path of the working medium,coopcrate with a mating surface on the compressor stator which surroundsthe tips of the blades to form a gas seal between therotor and stator.The tips of the compressor vanes, which extend radially inward from thecompressor case into the flow path of the working medium, cooperate witha corresponding surface on the rotor to form a gas seal between therotor and the stator. The aerodynamic performance of the compressor ishighly dependent on the clearances between the rotor and the stator atthe .blade and vane tips. Even a slight reductionin tip clearance canimprove the aerodynamic performance of the compressor significantly.

The larger the flow path diameter of the gas turbine, the more difficultit becomes to maintain acceptable tip clearances. The. sealing surfacesof the rotor and the arc circumscribed by the rotating blade tips areheld concentric with the axis of the rotor within very limitedtolerances tomaintain rotor balance. The compressor stator does notstructurally require this precision balancingand concentricitytolerances of the stator are, therefore, generally relaxed to reducemanufacturing cost. The extent to which these tolerances are relaxed,directly affects the concentricity of the blade and vane tips with theircorresponding sealing surfaces.

In an ideal condition all sealing surfaces of the rotor are concentricwith their corresponding vane tips, and all sealing surfaces of thestator are concentric with their corresponding blade tips. Thisconstruction permits a minimum clearance between the rotating andstationary parts. As eccentricity between the stationary androtatingparts is introduced the clearances must be increased to preventdestructive contact between the rotor and the stator during operation ofthe engine.

Additional clearance between the cooperating sealing surfaces of therotor and stator is also provided to allow for distortion of thecompressor case under transient thermal conditions. A compressor casehavinga nonuniform mass distributed about its circumference, such as anaxially split case, experiences distorted thermal growth due to theconcentration of mass in the flange areas..As the case is exposed tochanging thermal environments the areas of the case having low massconcentration expand more rapidly than those areas of the case havinghigh mass concentration due to the time differential required for theseareas-to reach steady state thermal conditions.

Most axial flow gas turbines have a double compressor case comprising-anouter case which provides structural support to the engine bearings andan inner case which radially defines the flow path of the working mediumand which supports the compressor vanes extending from the inner wallinto the flow path. The inner case comprises a series of annular ringsbolted together. Alternate rings support rows of stator vanes and arejoined by an intermediate ring which is the sealing surface opposing thecorresponding row of rotor blade tips. The joining features of each ringare formed with respect to the axis of each individual ring withinlimits of a concentricity tolerance. As adjacent rings are joinedtogether concentricity tolerances can accumulate so that, potentially,the misalignmentbetween the first and the last stages of the statorassembly can be excessive. Consequently design clearance between therotor and the stator assemblies is increased to accommodate for thispotential misalignment.

Other gas turbines, particularly those used in industrial applications,have a single compressor case which is split axially and joined bylongitudinal flanges on opposing halves of the compressor case. The massdistribution about the circumference of the compressor case isnonuniform and causes eccentric distortion of the compressor case duringtransient thermal conditions.

. Components supported by the areas having a high mass concentrationassume in a radially inward position with respect to those componentssupported by an area having a low mass concentration as the casetemperature is increasing and assume a radially outward position as thecase temperature is decreasing. Sufficient clearance between the bladeand vane tips and the corresponding sealing surfaces is provided toprevent destructive contact between the rotor and the stator in .theareas where the compressor case has greater mass.

A single compressor case with uniform mass distribution has thepotential for high aerodynamic performance. Although adequate mechanicalsupport of the vanes from such a case is difficult. The vanes arenecessarily cantilevered from the case and, therefore, tend to vibrateduring operation of the engine. Vanes which are rigidly mounted to thecompressor case have a limited lifetime since they tend to crack orfracture due to vibratory stresses in the area where the airfoil isjoined to a supporting structure. In addition, a single-wall caserequires vane loading slots into which the vanes are inserted andrepositioned circumferentially about the compressor case. The loadingslot region frequently experiences excessive stress concentration in theareas where small radii interrupt uniform stress patterns.

In US. Pat. No. 2,857,093, Warnkin discloses a plurality of stator vaneswhich are assembled on an arcuate segment and, subsequently, mounted ina circumferential track of an axially split compressor case. Eachcompressor vane has a wedge shaped root which is in; serted through anaperture in the arcuate segment where it is held firmly in position.

In US. Pat. No. 2,928,586 Hart describes a stator for a multistage axialflow compressor having a cylindrical casing wherein spacer rings areassembled between rows of blades and are held in place by boltspenetrating the case to engage each ring. Two axially adjacent spacerrings form a tee shaped retainer which engages the correspondinglyshaped base of each compressor vane. In Hart the potential misalignmentdue to concentricity buildup is increased over a single wall case withintegrally mounted vanes.

Efforts are continuing toward improvements in aerodynamic performancewhile maintaining the structural integrity of compressor statorsincluding efforts to nondestructively dissipate vibrational energy fromthe vanes.

SUMMARY OF THE INVENTION A primary object of the present invention is toimprove the structural integrity of the compressor case and compressorvanes of a gas turbine engine. A further object of the present inventionis to improve the aerodynamic performance of the compressor.

According to the present invention a plurality of compressor vaneshaving a retaining slot in the base portion of each vane is slidablymounted on an arcuate retainer having a correspondingly shaped crosssection, the vanes extending from the arcuate retainer in a radiallyinward direction toward the center of curvature of the arc; a pluralityof arcuate retainers and the vanes mounted thereupon comprise a set ofvanes for a single compressor stage and are bolted into acircumferential retaining track of uniform cross section which has beenmachined into the inner wall of the compressor case, one or more boltspenetrating the case from the outer wall engage each arcuate retainer.

A primary feature of the present invention is the dissipative transferof vibrational energy from the vanes to the working medium and to thecompressor case. An additional feature is the one piece compressor casehaving circumferential tracks of uniform cross section for retainingdetachable vane assemblies.

Principal advantages of the present invention are the ability of thestator assembly to dissipate vibrational energy without cracking thestator vanes and the ability of the compressor case to maintain uniformblade and vane tip clearances around the circumference of the rotor. Anadditional advantage of the present invention is the confinement of highstress concentrations to the vanes and to the arcuate retainers whichare easily replaceable and low cost parts.

The foregoing and other objects, features and advantages of the presentinvention will become more apparent in the light of the followingdetailed description of the preferred embodiments thereof as illustratedin the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a simplified elevation viewshowing an axial flow gas turbine engine;

FIG. 2 is a simplified cross-sectional view of a portion of thecompressor of the gas turbine engine shown in FIG. 1;

FIG. 3 is a section view of the compressor taken in the direction 3 asshown in FIG. 2;

FIG. 4 is a section view taken along the line 4-4 as shown in FIG. 3;

FIG. 5 is a section view of the compressor vane attachment under gaspressure loads; and

FIG. 6 is a section view of the compressor vane attachment under acondition of vibrational excitation in which the vibratory loads areopposite to and exceed the gas pressure loads.

DESCRIPTION OF THE PREFERRED EMBODIMENT The gas turbine 10 shown in FIG.1 is an axial flow engine having a multistage compressor 12 joined to amultistage turbine 14 by a combustor 16. Air is compressed in thecompressor, is mixed with fuel and burned in the combustion section toproduce hot gases which are expanded through a series of nozzles withinthe turbine section. The more air that an engine can 4 compress and usethe greater is the power or thrust that can be produced within theengine.

A portion of the multistage compressor is shown in cross section in FIG.2. A rotor assembly 18 comprises a plurality of compressor wheels 20which are separated axially by spacers 22. Each compressor wheelincludes a disk 24 and a plurality of blades attached thereto asrepresented by the single blade 26 on each wheel. Each blade has aplatform 28 at the base of an airfoil section 30. An axial gap betweenthe blade platforms of adjacent wheels is spanned by an inner air seal32. The rotor assembly is radially enclosed by a compressor stator 34comprising a plurality of vane stages 36 each mounted within acircumferential track 38 in a compressor case 40.

As is shown in FIG. 3 each vane stage comprises a plurality of vaneassemblies 42 which include one or more vanes 44, an arcuate retainer 46and a pair of end plates 48. Each vane assembly is held within thecircumferential track by one or more bolts 50 which penetrate the caseto engage the arcuate retainer.

Each compressor vane has an airfoil section 52, a base 54 includingretaining slot 56 and a tip 58 as shown in FIG. 4. An axial clearance 60is provided between the vane base and the compressor case and a radialclearance 62 is provided between the vane base and the arcuate retainer.As is shown in FIG. 5 each vane has a case bearing surface 64 which isopposed by a vane bearing surface 66 of the case and a retainer bearingsurface 68 which is opposed by a vane bearing surface 70 of theretainer.

During assembly of the compressor one or more vanes 44 are slidablymounted on each arcuate retainer 46, the tee shaped arcuate retainer ofthe preferred embodiment engaging the correspondingly shaped slot in thebase of each vane. An end plate 48 is affixed to each end of the arcuateretainer to trap the vanes on the retainer. A plurality of vaneassemblies is bolted into each circumferential track to form eachcompressor vane stage 36, each assembled vane extending radia'lly inwardacross the flow path of the working medium. The end plates perform theadditional function of preventing circumferential movement of the vanesabout the track during operation of the engine.

The number of vanes mounted within each vane assembly is variedaccording to the size and weight of the individual components. Includinga large number of vanes in each vane assembly lessens the number ofsteps required to assemble a complete vane stage in the compressor case.Including a smaller number of vanes in each vane assembly reduces theweight of the assembly and makes it more easily mountable within thecompressor case. In one embodiment a single vane stage comprises fivevane assemblies which weigh approximately thirty pounds each and includefourteen vanes. As many as ten vane assemblies are commonly used.

The number of vanes on the vane assembly to be last assembled is limitedby the cord length of its arcuate retainer. The cord length of the lastarcuate retainer must be smaller than the distance between the tips 58of the vanes through which the vane assembly passes as it is positionedinto the circumferential track from a radially inward direction. In theembodiment just described the fifth vane assembly is split and includesone vane assembly having thirteen vanes and one assembly having a singlevane as shown in FIG. 3. The vane assemblies are bolted into thecircumferential track by vane assembly comprising a'single vane ismounted within a circumferential track in the same manner as vaneassemblies having a plurality of vanes.

One of the significant aspects ofa compressor constructed in accordancewith the present invention is the damping of vibratory energy in thecompressor vanes during engine operation. The axial clearances 60between the base of each vane and the compressor case, and a radialclearance 62 between the base of each vane and the corresponding arcuateretainer permit limited movement of the vanes after the arcuate retainerof each vane assembly is secured to the compressor case. In a typicalembodiment both the radial and axial clearances are one thousandth tothirteen thousandths of an inch. During operation'of the compressor thevanes are pressure loaded and assume a canted position as shown in FIG.5. The vanes tilt toward the front or low pressure end of the compressoruntil the retainer bearing surface of the vane 68 comes to rest againstthe vane bearing surface of the retainer 70 and simultaneously the casebearing surface of the vane 64 comes to rest against the vane bearingsurface of the case 66. Within the normal operating ranges of theengine, inherent vibrational loads cyclically exceed the static pressureloads on the vane causing the vane to tilt from its forward position toa rearward position as shown in FIG. 6; Rearward movement of the vane isop-- posed by the pressure loading forces which cushion the airfoilsurface to dissipate vibrational energy from the vane. Additionally,friction damping occurs between the side bearing surfaces of adjacentvanes and between the bearing surfaces on each vane which are in contactwith the retainer or the case.

In contrast to the present invention, vibrational energy is mostcommonly removed from the compressor vanes of gas turbine enginesthrough a rigidly fixed attachment joining the vane and to the case.With this construction the life of the vane is shortened becausevibratory stresses concentrate at the juncture between the vane airfoiland the vane base. The cumulative vibratory stresses at this junctureultimately crack or fracture the vanes. Vanes attached in accordancewith the present invention are not rigidly affixed to the case and donot experience excessive vibratory stresses.

The compressor case has essentially u-shaped tracks machined into thecircumferential inner wall. The absence of vane loading slots allowscase stresses to uniformly distribute around the circumference of thecase thereby maximizing the case life. Stress concentrations within thestator do occur at the internal structural corners of the retainer andthe vane base. However, the retainers and the vanes are easily replacedat minimal cost when structural cracks appear.

Movement of the vanes relative to the case and the arcuate retainer cancause wear along the surfaces of contact. To prevent excessive wear thebearing surfaces are treated with a hardfacing material. Theconstruction shown in the preferred embodiment has a simple geometrywhich permits the application of hardfacing material to the bearingsurfaces.

Significant aerodynamic improvements result from a compressorconstructed in accordance with the present invention. In one embodimentan increase in the blade and vane tip clearance of ten thousandthsthroughout the length of the compressor decreases the compressorefficiency by one percent. A common design goal is the 6 control of thetip clearances to within one percent of the 'spanwidth of the airfoilsections which is for one typical embodiment a thirty eight thousandthsnominal clearance between each vane or blade tip and its correspondingsealing surface at a diameter of approximately fifty inches.

Control of the tip clearance requires control of the distortion of thecompressor case and control of the concentricity of mating surfaces withrespect to a common axis. Control of the compressor case distortion isachieved by use of a nonsplit compressor having a uniformcross-sectional area about its circumference at any axial position. Theuse of a nonsplit compressor case allows a uniform cross section byeliminating the mass concentration of the flanges joining a splitcompressor case. The areas of a split case which have high mass, such asthe flange areas, exhibit a retarded rate of thermal response. Anonuniform thermal response distorts the sealing surfaces of the case atthe blade tips from a circular configuration, and alters the radiallyinward position of effected compressor vanes. In a split case the tipclearances must be adjusted to compensate for the range of thermalresponse rather than a single thermal response as in the preferredembodiment.

A second principal problem in holding minimum tip clearances is thebuildup of concentricity tolerances between opposing compressor parts.Most gas turbines in use today utilize stator constructions of thedouble case type wherein the inner case supports the compressor vanesand the outer wall provides structural support to the engine bearings.The inner case supporting the vanes comprises a plurality ofcylindrically shaped vane supports placed in axially adjacent positionsand bolted together. Each cylindrical support is machined toconcentricity tolerances in relation to its own axis. As the supportcylinders are bolted together the potential concentricity misalignmentincreases from the first support to the last support. In the compressorcase of the present invention each circumferential track is machined inrelation to the same axis which is the axis of the compressor case.There is, therefore, a single uniform concentricity tolerance at allaxial stages of the compressor. Although, the compressor rotor issubject to the same type of tolerance buildup experienced with thedouble compressor case, the magnitude of the tolerance buildup is not asgreat because rotor concentrici; ties are already accurately controlledto maintain rotor balance.

In addition, elimination of the inner case of the compressor reduces thestator cost significantly and in the embodiment described by about onethird. The single case construction has a reduced number of pieces and,therefore, has reduced assembly complexity. The single case constructionis lighter than the double case construction and can be mass balancedfor improved blade tip clearance.

In mass balancing, the mass of the compressor case at any axial positioncan be increased to match the predicted thermal response of the rotor atthat axial position. Although the thermal response of a compressorhaving a double case construction may be similarly controlled, thegeometry is more complex and accurate distortion prediction is moredifficult.

Although the invention has been described with respect to a preferredembodiment having a pair of end plates attached one to each end of eacharcuate retainer, a single end plate attached to one end of each arcuateretainer is equally effective in preventing the 7 circumferentialrotation of the vanes around the track in the inner wall of the case.

Although the invention has been shown and described with respect to apreferred embodiment thereof, it should be understood to those skilledin the art that the foregoing and other changes and omissions in theform and detail thereof can be made therein without departing from thespirit and the scope of the invention.

We claim:

1. In a gas turbine engine, a compressor stator including a plurality ofvane assemblies detachably mounted within a circumferential track in theinner wall of the case of the compressor wherein each vane assemblycomprises:

an arcuate retainer which is attached to the case and which iscoextensive with the case over a segment of the case circumference;

at least one vane having a base including a retaining slot which isslidably engaged by the arcuate retainer, the vane extending radiallyinward from the retainer toward the center of curvature of the retainer;and

a first end plate attached to one end of the arcuate retainer forpreventing the circumferential rotation of the vanes around the track inthe inner wall.

2. The invention according to claim 1 further including a second endplate attached to the end of the arcuate retainer opposite said firstend plate for trapping the vanes on the arcuate retainer.

3. A stator construction for an axial flow compressor having means fordamping vane vibration including a compressor case having, in the caseinner wall, a circumferential track containing a plurality of vaneassemblies each of which comprises an arcuate retainer slidably engagedto a slot in the base of the stator vanes which are attached to andextend radially inward from the arcuate retainer toward the center ofcurvature of the retainer, and a first end plate attached to one end ofthe retainer to prevent circumferential movement of the vanes about thecompressor case, each of said vanes having an axial and radial clearancebetween the vane base and the compressor case which permits movement ofthe vane under the influence of vibrational excitation to dissipativelytransfer vibrational energy from the blade to the working medium bypressure load forces on the airfoil and to the compressor case byfriction forces between the vanes, case and retainer.

4. The invention according to claim 3 wherein the axial and radialclearances between the vane base and the compressor case are in therange of one to thirteen thousandths of an inch.

5. The invention according to claim 4 wherein the slot in the base ofeach vane has a tee shaped cross section.

6. The invention according to claim 5 wherein the compressor case has asubstantially uniform cross-sectional area at any axial position alongthe length of the case.

7. The invention according to claim 6 wherein the number of vaneassemblies comprising a single compressor stage is between five and ten.

8. The invention according to claim 7 wherein at least one assemblyincludes no more than one vane 9. The invention according to claim 8further including a second end plate attached to the end of the arcuateretainer opposite said first end plate for trapping the vanes on thearcuate retainer.

1. In a gas turbine engine, a compressor stator including a plurality ofvane assemblies detachably mounted within a circumferential track in theinner wall of the case of the compressor wherein each vane assemblycomprises: an arcuate retainer which is attached to the case and whichis coextensive with the case over a segment of the case circumference;at least one vane having a base including a retaining slot which isslidably engaged by the arcuate retainer, the vane extending radiallyinward from the retainer toward the center of curvature of the retainer;and a first end plate attached to one end of the arcuate retainer forpreventing the circumferential rotation of the vanes around the track inthe inner wall.
 2. The invention according to claim 1 further includinga second end plate attached to the end of the arcuate retainer oppositesaid first end plate for trapping the vanes on the arcuate retainer. 3.A stator construction for an axial flow compressor having means fordamping vane vibration including a compressor case having, in the caseinner wall, a circumferential track containing a plurality of vaneassemblies each of which comprises an arcuate retainer slidably engagedto a slot in the base of the stator vanes which are attached to andextend radially inward from the arcuate retainer toward the center ofcurvature of the retainer, and a first end plate attached to one end ofthe retainer to prevent circumferential movement of the vanes about thecompressor case, each of said vanes having an axial and radial clearancebetween the vane base and the compressor case which permits Movement ofthe vane under the influence of vibrational excitation to dissipativelytransfer vibrational energy from the blade to the working medium bypressure load forces on the airfoil and to the compressor case byfriction forces between the vanes, case and retainer.
 4. The inventionaccording to claim 3 wherein the axial and radial clearances between thevane base and the compressor case are in the range of one to thirteenthousandths of an inch.
 5. The invention according to claim 4 whereinthe slot in the base of each vane has a tee shaped cross section.
 6. Theinvention according to claim 5 wherein the compressor case has asubstantially uniform cross-sectional area at any axial position alongthe length of the case.
 7. The invention according to claim 6 whereinthe number of vane assemblies comprising a single compressor stage isbetween five and ten.
 8. The invention according to claim 7 wherein atleast one assembly includes no more than one vane.
 9. The inventionaccording to claim 8 further including a second end plate attached tothe end of the arcuate retainer opposite said first end plate fortrapping the vanes on the arcuate retainer.